1. Field of the Invention
The invention relates to a method of forming a semiconductor device, and more particularly to a method of forming a trench gate metal-oxide-semiconductor field effect transistor (MOSFET).
2. Description of Related Art
Trench MOSFET has been widely applied in power switch devices, such as power supplies, rectifiers, low-voltage motor controllers, or so forth. In general, the trench MOSFET is often resorted to a design of vertical structure to enhance the device density. In a power MOSFET, each drain region is formed on the back-side of a chip, and each source region and each gate are formed on the front-side of the chip. The drain regions of the transistors are connected in parallel so as to endure a considerable large current.
A working loss of the trench MOSFET may be divided into a switching loss and a conducting loss, wherein the switching loss caused by the input capacitance Ciss is going up as the operation frequency is increased. The input capacitance Ciss includes a gate-to-source capacitance Cgs and a gate-to-drain capacitance Cgd. When the gate-to-drain capacitance Cgd is decreased, the switching loss is accordingly reduced.
A conventional method is to fill in the trench with an insulating layer, and then to remove the insulating layer with an etching back process, so that a thick oxide layer is formed at the bottom of the trench to lower the gate-to-drain capacitance Cgd. However, the above method is very difficult and requires a precise control in order to avoid a channel offset.